Many structural parts (e.g. many internal fittings of automobiles; discs) and objects of daily use (e.g. beverage bottles) consist substantially of electrically insulating materials. This includes known polymers, such as polyvinyl chloride, polypropylene etc., but also ceramics, glass and other mineral materials. In many cases the insulating effect of the structural part is desired (e.g. in the case of housings of portable computers). However, there is often also a need to apply an electrically conducting surface or structure to such structural parts or objects, in order for example to integrate electronic functions directly into the structural part or the object.
Further requirements placed on the surface of articles of daily use and their material include as great an artistic freedom as possible in the design and configuration, positive mechanical properties (e.g. high impact strength), as well as specific optical properties (e.g. transparency, gloss, etc.), which are achieved in different degrees particularly by the materials listed above by way of example.
There is therefore the need to obtain the positive properties of the material and, specifically, to produce an electrically conducting surface. In particular the optical transparency and gloss are in this connection technically demanding. These can be achieved only in three ways. Either the substrate material itself is specifically made electrically conducting, without thereby adversely affecting its mechanical and optical properties, or a material is used that is conducting but is not visually recognisable by the human eye and can easily be applied in a targeted manner to the surface of the substrate, or a conducting material is used, which although itself is not transparent, can however be applied by means of a suitable process to the surface in such a way that the resulting structure is in general not perceivable by the human eye without the assistance of optical aids. In this way the properties of gloss and transparency of the substrate are not affected.
In general any structure which, when applied to a two-dimensional surface does not exceed a characteristic length of 20 μm in one of its two dimensions on the substrate plane, is regarded as visually non-recognisable. In order reliably to exclude any influencing of the surface recognition, structures in the submicron range (i.e. with a line width of ≦1 μm) are particularly desirable.
A large number of methods exist for applying in particular conducting material to surfaces. In particular conventional printing methods, such as screen printing or ink jet printing, are suitable for this purpose. Corresponding formulations for conducting materials—also termed inks—already exist particularly for these printing techniques, which in conjunction with the methods enable conducting structures to be formed on the surface.
Whereas screen printing methods on account of the very small available mesh width of the printing screen are in principle not able to produce structures with an optical resolution of less than 1 μm, ink jet printing methods for example would theoretically be suitable for this purpose, since the dimensions of the resulting structure on the substrate in the case of ink jet printing methods directly correlate to the nozzle diameter of the printing head that is used. However, in this connection the characteristic length of the minimal dimension of the resulting structure is as a rule larger than the diameter of the employed nozzle head [J. Mater. Sci 2006, 41, 4153; Adv. Mater 2006, 18, 2101]. Nevertheless, in principle structures with a line width of less than 1 μm could be produced if printers with nozzle openings of significantly less than 1 μm can be used. However, this is not feasible in practice since with increasing reduction of the nozzle diameter the requirements on the inks that can be used become much more stringent. Should the employed ink contain particles, then their mean diameter would have to match the reduction in the nozzle diameter, which in principle already excludes all inks with particles of size ≧1 μm. Furthermore, the requirements placed on the rheological properties of the ink (e.g. viscosity, surface tension, etc.) so that it can still be used for the printing head increase. In many cases these parameters cannot however be adjusted separately from the behaviour (e.g. spreading and adherence) of the ink on the respective substrate, which means that the ink and printing method combination cannot be used to produce conducting structures in this size range.
One method with which alternatively structures of size less than 1 μm can be produced on polymer surfaces is the so-called hot stamping method. By means of this method circular surface structures with a diameter of ca. 25 nm have already been produced [Appl Phys Lett 1995, 67, 3114; Adv Mater 2000, 12, 189]. The disadvantage of hot stamping however is that the structural shape is restricted to the shape of the stamping punch or stamping roller that is used in each case. A free choice in the configuration of the structure is not possible with this method. Particularly thin fibres, which potentially could also be applied to the surface of a suitable substrate, can be produced by means of a method that has become established under the name “electrospinning”. In this way it is possible by using a spinnable material to produce fibres of a few nanometres in diameter [Angew Chem 2007, 119, 5770-5805].
Electrospun fibres are however obtained only in the form of large, disordered fibre mats. Up to now ordered fibres can however be obtained only by spinning on a rotating roller [Biomacromolecules, 2002, 3, 232]. It is also known that in principle electrically conducting fibres can be spun by means of “electrospinning”. A corresponding conducting material for such an application utilising the conductivity of carbon nanotubes is also known [Langmuir, 2004, 20(22), 9852].
In US2001-0045547 methods and materials are disclosed, with which conducting fibre mats can be obtained.
A targeted deposition of non-conducting fibres on planar surfaces has also been achieved by reducing the distance between the spinning head and the substrate [Nano Letters, 2006, 6, 839].
Up to now no electrically conducting structures with a specific arrangement on a substrate surface have been produced by means of electrospinning.
In US2005-0287366 a method and a material are disclosed, by means of which conducting fibres can be produced. The method includes electrospinning at an interspacing of about 200 mm, with the result that disordered fibre mats are likewise obtained. The material is a polymer that is made electrically conducting by further post-treatment steps, including a thermal treatment. A targeted orientation and application of the resultant fibres to a substrate is not disclosed.
The object of the present invention is accordingly to develop a process with which, by using the electrospinning technique, conducting structures that are visually not directly recognisable by the human eye can be specifically produced on a surface.